BACKGROUND & AIMS: CD8(+) T cells that produce interleukin (IL)-17 (Tc17 cells) promote inflammation and have been identified in tumors. We investigated their role in the pathogenesis of gastric cancer. METHODS: We used flow cytometry analyses to determine levels and phenotype of Tc17 cells in blood and tumor samples from 103 patients with gastric cancer. We performed multivariate analysis to identify factors associated with overall survival using the Cox proportional hazards model. CD8(+) T cells and monocytes were isolated and cocultured in an assay for induction of Tc17 cells. Tumor cells and myeloid-derived suppressor cells (MDSCs) were isolated and used in assays of Tc17 cell function. RESULTS: Tc17 cells with distinct cytokine and functional profiles were found in gastric tumor samples from patients. The percentage of Tc17 cells increased with tumor progression and was associated with overall survival time. Tumor-activated monocytes secreted IL-6, IL-1β, and IL-23, which promoted development of Tc17 cell populations. Supernatants from cultured Tc17 cells induced production of the chemokine CXCL12 by tumor cells; this promoted CXCR4-dependent migration of MDSCs and impaired functions of anti-tumor CD8(+) cytotoxic T cells via a cell contact-dependent mechanism. CONCLUSIONS: Percentages of Tc17 cells in gastric tumors are associated with survival times of patients. These cells promote chemotaxis of MDSCs, which might promote tumor progression.